Proto2: MessageSet wire format (deferred)

protobuf-go's protoreflect refuses to load a FileDescriptor declaring 'option message_set_wire_format = true;', calling it a removed proto1 feature. We strip the four MessageSet-flavored nested messages from our vendored test/conformance/proto/test_messages_proto2.proto so the rest of the schema compiles. Real support would require forking protoreflect; deferred until a real Tarantool consumer asks. Track here so we don't lose the context.